The present invention relates to a magnetic resonance signal acquiring method and apparatus, a recording medium and a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance signal acquiring method and apparatus for effecting RF (radio frequency) excitation for preparation on spins within an object, followed by effecting RF excitation again and acquiring a navigator echo, and effecting RF excitation for preparation on the spins within the object, followed by effecting RF excitation again and acquiring an imaging echo, a recording medium recorded with a program for causing a computer to perform such a magnetic resonance signal acquiring function, and a magnetic resonance imaging apparatus comprising such magnetic resonance signal acquiring means.
In a magnetic resonance imaging (MRI) apparatus, an object to be imaged is carried into an internal space of a magnet system, i.e., a space in which a static magnetic field is generated; a gradient magnetic field and a high frequency magnetic field are applied to generate magnetic resonance signals from spins within the object; and a tomographic image is constructed based on the received signals. The magnetic resonance signal for use in constructing the tomographic image is sometimes referred to as an imaging echo.
Since the spin frequency is proportional to the magnetic field strength, the spin frequency varies as the static magnetic field strength varies. If the temporal variation in the spin frequency is considerably large, the current value of the spin frequency is determined before effecting RF excitation for generating an imaging echo, and the RF excitation for generating the imaging echo is effected by the high frequency magnetic field having a frequency equal to the current value of the spin frequency.
In order to evaluate the current value of the spin frequency, the spins are RF-excited separately to measure an FID (free induction decay) signal. The FID is sometimes referred to as a navigator echo. The spin frequency is determined from the navigator echo by a calculation.
Before effecting the RF excitation for the imaging echo, RF excitation for bringing the state of spins within the object to a predetermined state may be effected. Such RF excitation is sometimes referred to as preparation. The RF excitation signal for the preparation is sometimes referred to as a preparation pulse.
The preparation includes, for example, spatial pre-saturation for saturating spins in a desired region within the object, and chemical saturation for saturating spins contained in a particular molecular structure.
Moreover, magnetization transfer for reducing the signal strength from tissues having a high protein concentration, and inversion for inverting spins in order to perform imaging according to an IR (inversion recovery) technique are also included in the category of preparation. Preparation is performed also in acquiring the navigator echo.
FIG. 1 is a time chart of navigator echo collection accompanied by preparation, a calculation on the navigator echo and imaging echo collection accompanied by preparation. The timing of gradient magnetic field application is omitted in the drawing.
As shown, the navigator echo collection accompanied by preparation and the imaging echo collection accompanied by preparation are separately performed during two consecutive periods. The length of each period is TR (repetition time).
In the earlier period, a preparation pulse such as, for example, a spatial pre-saturation pulse is applied at a time t1. Next, at a time t2, RF excitation is effected for generating a navigator echo. Next, from a time t3 to a time t4, navigator echo collection is performed.
Next, from a time t5 to a time t6, a calculation is performed based on the collected echo data. The spin frequency is determined by the calculation. The result of the calculation is reflected on the frequency of the RF excitation in the later period.
In the later period, a preparation pulse such as, for example, a spatial pre-saturation pulse is applied at a time t7. The frequency of the preparation pulse is made equal to the spin frequency determined by the calculation in the earlier period.
Next, at a time t8, RF excitation is effected for generating an imaging echo. The frequency of the RF excitation is made equal to the spin frequency determined by the calculation in the earlier period. Next, from a time t9 to a time t10, imaging echo collection is performed.
By repeating the above operation, imaging echoes for a plurality of views are sequentially collected. Each of the imaging echoes is given different phase encoding for each view by the gradient magnetic field, which is omitted in the drawing.
In the above operation, the calculation on the navigator echo should be completed within an interval after the navigator echo collection up to the beginning of the next period. However, this interval is short because it is the remainder of the period TR after subtraction of the time required for the preparation, RF excitation and navigator echo collection, and the time available for performing the calculation is insufficient. Accordingly, the calculation must be sped up to complete the calculation within the interval, or, if this is not possible, the period TR must be extended.
Therefore, it is an object of the present invention to provide a magnetic resonance signal acquiring method and apparatus in which the time available for a calculation on a navigator echo is large, and a recording medium recorded with a program for causing a computer to perform such a magnetic resonance signal acquiring function, and a magnetic resonance imaging apparatus comprising such magnetic resonance signal acquiring means.
(1) The present invention, in accordance with one aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring method that executes the steps of effecting RF excitation for preparation on spins within an object, followed by effecting RF excitation again and acquiring a navigator echo; effecting RF excitation for preparation on the spins within the object, followed by effecting RF excitation again and acquiring an imaging echo; and performing a calculation on said acquired navigator echo to adjust the frequency of said RF excitation based on the result of the calculation, which method is characterized in comprising: effecting the RF excitation on the spins within the object and acquiring the navigator echo, and thereafter effecting the RF excitation for preparation during one of two consecutive periods; and effecting the RF excitation on the spins within the object and acquiring the imaging echo, and thereafter effecting the RF excitation for preparation during the other of the two consecutive periods.
In this aspect of the invention, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for preparation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for preparation is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(2) The present invention, in accordance with another aspect for solving the aforementioned problem, is the magnetic resonance signal acquiring method as described regarding (1), characterized in that said preparation is spatial pre-saturation.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for spatial pre-saturation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for spatial pre-saturation is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(3) The present invention, in accordance with still another aspect for solving the aforementioned problem, is the magnetic resonance signal acquiring method as described regarding (1), characterized in that said preparation is chemical saturation.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for chemical saturation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for chemical saturation is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(4) The present invention, in accordance with still another aspect for solving the aforementioned problem, is the magnetic resonance signal acquiring method as described regarding (1), characterized in that said preparation is magnetization transfer.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for magnetization transfer is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for magnetization transfer is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(5) The present invention, in accordance with still another aspect for solving the aforementioned problem, is the magnetic resonance signal acquiring method as described regarding (1), characterized in that said preparation is inversion.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for inversion is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for inversion is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(6) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance signal acquiring apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, characterized in that the apparatus comprises: preparation means for effecting RF excitation for preparation on spins within an object; echo acquiring means for effecting RF excitation on the spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of said RF excitation based on the result of said calculation; and control means for controlling said echo acquiring means to perform the navigator echo acquisition, and thereafter controlling said preparation means to effect the RF excitation for preparation during one of two consecutive periods, and for controlling said echo acquiring means to perform the imaging echo acquisition, and thereafter controlling said preparation means to effect the RF excitation for preparation during the other of the two consecutive periods.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for preparation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for preparation is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(7) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium recorded in a computer-readable manner with a program for causing a computer to execute the functions of effecting RF excitation for preparation on spins within an object, followed by effecting RF excitation again and acquiring a navigator echo; effecting RF excitation for preparation on the spins within the object, followed by effecting RF excitation again and acquiring an imaging echo; and performing a calculation on said acquired navigator echo to adjust the frequency of said RF excitation based on the result of the calculation, which recording medium is characterized in that the program causes the computer to effect the RF excitation on the spins within the object and acquire the navigator echo, and thereafter effect the RF excitation for preparation during one of two consecutive periods; and effect the RF excitation on the spins within the object and acquire the imaging echo, and thereafter effect the RF excitation for preparation during the other of the two consecutive periods.
In the invention of this aspect, since a program recorded on the recording medium causes a computer to perform functions of effecting the RF excitation on spins within an object and acquiring the navigator echo and thereafter effecting the RF excitation for preparation during one of two consecutive periods, and effecting the RF excitation on the spins within the object and acquiring the imaging echo and thereafter effecting the RF excitation for preparation during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
(8) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring magnetic resonance signals from an object using a static magnetic field, a gradient magnetic field and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signals, characterized in that means for acquiring said magnetic resonance signals comprises: preparation means for effecting RF excitation for preparation on spins within an object; echo acquiring means for effecting RF excitation on the spins within the object and acquiring a navigator echo, and for effecting RF excitation on the spins within the object and acquiring an imaging echo; calculating means for performing a calculation on said acquired navigator echo; frequency adjusting means for adjusting the frequency of said RF excitation based on the result of said calculation; and control means for controlling said echo acquiring means to perform the navigator echo acquisition, and thereafter controlling said preparation means to effect the RF excitation for preparation during one of two consecutive periods, and for controlling said echo acquiring means to perform the imaging echo acquisition, and thereafter controlling said preparation means to effect the RF excitation for preparation during the other of the two consecutive periods.
In the invention of this aspect, since the RF excitation is effected on spins within an object and the navigator echo is acquired and thereafter the RF excitation for preparation is effected during one of two consecutive periods, and the RF excitation is effected on the spins within the object and the imaging echo is acquired and thereafter the RF excitation for preparation is effected during the other of the two consecutive periods, the interval from the navigator echo acquisition to the beginning of the next period is long, and the time available for the calculation on the navigator echo is long.
Therefore, the present invention can provide a magnetic resonance signal acquiring method and apparatus in which the time available for a calculation on a navigator echo is long, and a recording medium on which a program is recorded for causing a computer to perform such a magnetic resonance signal acquiring function, and a magnetic resonance imaging apparatus comprising such magnetic resonance signal acquiring means.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.